Organic el device and organic el panel

ABSTRACT

A transparent electrically conductive film comprising one of In 2 O 3 —ZnO, In 2 O 3 —SnO 2 , ZnO, and SnO 2  is provided on a surface of a metal electrode of an organic EL (electroluminescent) device on the light-emitting layer side, and the thickness of this transparent electrically conductive film is set such as to satisfy the following equation, where L is the optical distance from the organic light-emitting layer to the metal electrode, and λ is the emission wavelength, whereby light reflected by the metal electrode is made to undergo interference and thus strengthen itself in the device; as a result, there are provided an organic EL device and an organic EL panel using the same, according to which the external quantum efficiency can be improved with no accompanying deterioration in the brightness, and moreover the contrast can be improved:
 
 L =( 2   n+   1 )λ /4  ( n=   0, 1, 2 , . . . ).

CROSS-REFERENCE TO RELATED APPLICATION

The present application incorporates herein by reference the entirecontents and disclosure of the corresponding PCT applicationPCT/JP2003/007565. Also incorporated herein by reference are thecontents and disclosure of the corresponding earlier Japaneseapplication JP PA 2002-074993.

TECHNICAL FIELD

The invention relates to an organic EL (electroluminescent) device andan organic EL panel, and more specifically relates to an organic ELdevice and an organic EL panel using the same, according to which theexternal quantum efficiency can be improved with no accompanyingdeterioration in the brightness, and moreover the contrast can beimproved.

REVIEW OF RELATED TECHNOLOGY

Ever since the announcement by Tang in 1987 of a high-efficiency organicEL device having a two-layer laminated structure (C. W. Tang et al.,Appl. Phys. Lett., 51, 913 (1987)), various organic EL devices have beendeveloped, and some of these have already been put into practical use.

FIG. 4 is a drawing for explaining the structure of a conventionalorganic EL device; the device has a constitution in which a holetransport layer 42, a hole injection layer 43, a light-emitting layer44, an electron transport layer 45, and an electron injection layer 46are formed in this order on a transparent electrode 41, which is ananode, and a metal electrode 47, which is a cathode, is provided on theelectron injection layer 46.

In FIG. 4, the hollow arrow indicates light direction.

The quantum efficiency of an organic EL device having a structure asshown in FIG. 4 is thought of as follows. First, holes and electronsarriving from the anode and the cathode form electron-hole pairs in thelight-emitting layer, thus becoming excitons having light-emittingability; the probability of production of such a light-emitting excitonis approximately 25%. The efficiency χ of extracting light produced inthe light-emitting layer out to the outside of the device, on the otherhand, is given by the following equation, where n is the refractiveindex of the light-emitting layer:χ=1/(2n)  (1)

Generally, the refractive index of the light-emitting layer is 1.6, andhence the efficiency χ of extracting out light is approximately 20%. Thetheoretical external quantum efficiency limit is thus given by theproduct of the probability of production of a light-emitting exciton(approximately 25%) and the efficiency of extracting out light(approximately 20%), which is approximately 5%.

However, the external quantum efficiency of an actual organic EL deviceis low, at approximately 3%, which is about 60% of the theoreticalvalue, and hence a problem arises in that if the current passed throughthe device is increased to extract light of a fixed brightness to theoutside, then deterioration of the brightness will progress, and inaddition the energy consumption will be increased.

Moreover, with actual panels, a problem of contrast in which the displaybecomes hard to see due to external light is a problem. One cause ofsuch a drop in the contrast is said to be the metal electrode reflectingexternal light.

In view of such problems, it is an object of the invention to provide anorganic EL device and an organic EL panel using the same, according towhich the external quantum efficiency can be improved with noaccompanying deterioration in the brightness, and moreover the contrastcan be improved.

SUMMARY OF THE INVENTION

The invention has been accomplished to solve the above problems. Theinvention can include an organic EL (electroluminescent) devicecomprising an organic EL light-emitting part including an organiclight-emitting layer, between a metal electrode and a transparentelectrode, characterized in that a transparent electrically conductivefilm is provided on a surface of the metal electrode on the organic ELlight-emitting part side, and the thickness of the transparentelectrically conductive film is set such as to satisfy the followingequation, where L is the optical distance from the organiclight-emitting layer to the metal electrode, and λ is the wavelength oflight emitted by the organic light-emitting layer:L=(2n+1)λ/4 (n=0, 1, 2, . . . )  (2)

Moreover, the invention can include the organic EL devices describedabove, characterized in that a material of the transparent electricallyconductive film is one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO, and SnO₂.

Moreover, the invention can be characterized in that the transparentelectrically conductive film has an impurity added thereto so as to becolored to a color the same as the color of the light emitted by theorganic EL light-emitting layer. Such an organic EL device can befurther characterized in that the organic EL light-emitting layer emitsblue light, the transparent electrically conductive film is constitutedfrom a material of one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂,containing an impurity of one of CuO, Co and Ti at a concentration ofnot more than 1%, and the transparent electrically conductive filmabsorbs blue light.

Moreover, the invention includes a monochrome panel or area color panel,characterized by comprising the organic EL device as set out above.

Moreover, can include a color conversion type color panel, characterizedby comprising the organic EL device as set out above, a blue monochromebacklight, and color-converting filters, wherein light other than bluelight is absorbed by the transparent electrically conductive film of theorganic EL device, and only blue monochrome light from the backlight isreflected by the metal electrode.

The invention as described above can be characterized in that theorganic EL light-emitting layer emits blue light, the metal electrodecomprises Zn, Mo or Cr, or an alloy thereof, and the metal electrodeabsorbs blue light. Moreover, such can include is a color conversiontype color panel, characterized by comprising the organic EL device asdescribed above, a blue monochrome backlight, and color-convertingfilters, wherein light other than blue light is absorbed by the metalelectrode, and only blue monochrome light from the backlight isreflected by the metal electrode.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a cross-sectional view for explaining an example of theconstitution of an organic EL device of the invention.

FIG. 2 is a cross-sectional view for explaining a second example of theconstitution of an organic EL device of the invention.

FIG. 3 is a cross-sectional view of a color conversion type color panelconstituted using an organic EL device of the invention.

FIG. 4, labeled “prior art,” is a cross-sectional view illustrating thestructure of a conventional organic EL device.

DETAILED DECRIPTION OF THE PREFERRED EMBODIMENTS

The following is a description of embodiments of the invention withreference to the drawings.

FIG. 1 is a drawing for explaining an example of the constitution of anorganic EL (electroluminescent) device of the invention formed on asubstrate; this organic EL device includes an organic EL light-emittingpart constituted from a plurality of organic layers including an organiclight-emitting layer, and specifically has a constitution in which ahole injection layer 12, a hole transport layer 13, a light-emittinglayer 14, an electron transport layer 15, and an electron injectionlayer 16 are formed in this order on a transparent electrode 11, whichis an anode. A transparent electrically conductive film 17 is providedon the electron injection layer 16, and a metal electrode 18, which is acathode metal layer, is provided on the transparent electricallyconductive film 17. Note that when constituting the organic EL device ofthe invention, a glass substrate may be provided on either thetransparent electrode 11 (the anode), or the metal electrode 18 (thecathode metal layer).

Of the light emitted from the light-emitting layer 14, the light emittedtoward the hole transport layer 13 side passes through the holetransport layer 13 and the hole injection layer 12 and is extracted tothe outside through the transparent electrode 11; this light isindicated by the straight, hollow, downward-pointing arrow in FIG. 1.The light emitted to the electron transport layer 15 side passes throughthe electron transport layer 15, the electron injection layer 16 and thetransparent electrically conductive film 17, and is reflected by themetal electrode 18 and thus returns into the device; this light isindicated by the reverse-curving or J-shaped hollow arrow in FIG. 1.Consequently, if this reflected light can be extracted to the outsidewithout being weakened in the device, then the external quantumefficiency can be improved.

That is, taking the thicknesses and refractive indices of the electrontransport layer 15, the electron injection layer 16 and the transparentelectrically conductive film 17 in the device to be d_(i) (i=1, 2, 3)and n_(i) (i=1, 2, 3) respectively, the optical distance L from thelight-emitting layer 14 to the metal electrode 18 will be given by thesum of the optical distances for these layers, i.e. by the followingequation.L=Σi n_(i)d_(i)  (4)

An optical distance is shown in FIG. 1 by the vertical, thin-linedouble-headed arrow.

When light is reflected at the interface between the metal electrode 18and the transparent electrically conductive film 17, the phase of thelight is reversed, and hence taking the wavelength of the light to be λ,a condition for the light strengthening itself in the device is:L=(2n+1)λ/4 (n= 0, 1, 2, . . . )  (5)

Because the metal electrode 18 is used as the cathode, and the electrontransport layer 15, the electron injection layer 16 and the transparentelectrically conductive film 17 are interposed between the metalelectrode 18 and the light-emitting layer 14, if design is carried outsuch that the optical distance accounted for by these layers satisfiesequation (5), then the external quantum efficiency can be improved.

However, the thickness of the electron injection layer 16 must be madelow at approximately 0.5 to 1 nm, and moreover if the thickness of theelectron transport layer 15 is made to be high then there will be aproblem of deterioration of the brightness of the device becomingmarked; consequently, with the organic EL device of the invention, thetransparent electrically conductive film 17 is provided between theelectron injection layer 16 and the metal electrode 18, and thethickness of the transparent electrically conductive film 17 is set suchthat the light reflected by the metal electrode 18 satisfies the aboveinterference condition, whereby the light can be extracted to theoutside without the strength of the light being weakened in the device,and hence the external quantum efficiency can be improved.

The method of setting the optical distance such that the externalquantum efficiency is maximized by adjusting the thickness of thetransparent electrically conductive film 17 in this way is not onlyeffective with a monochrome panel or area color panel that is made toemit light using a monochrome backlight, but moreover is particularlyeffective with a color panel that uses a color conversion method inwhich light emitted from a monochrome backlight is received bycolor-converting layers and converted into red, green and blue light.

Moreover, one practical problem of organic EL panels is a drop incontrast due to external light, and it has been ascertained that a causeof this is external light being directly reflected by the metalelectrode. According to equation (5), there is limited light ofwavelengths strengthened by interference, with only light of specificwavelengths being reflected; the strength of reflection of light ofwavelengths not satisfying equation (5) will thus be reduced, and henceit can be seen that the organic EL device of the invention contributesto improving the contrast of an organic EL panel.

To further improve the contrast, it is effective to form the transparentelectrically conductive film and the metal layer on top of one anotherto constitute a reflective layer, and color the transparent electricallyconductive film out of this reflective layer to the color of the emittedlight, thus producing a structure according to which light of colorsother than the color of the emitted light cannot be reflected, or makethe material of the metal layer be a material having a property ofabsorbing light of colors other than the color of the emitted light. Onecan thus envisage a method in which light of wavelengths not required tobe extracted out from the transparent electrode is absorbed by thelaminate of the transparent electrically conductive film and the metallayer, or a method in which this light is absorbed by the metal layermaterial. Note that, in this case, it is preferable to constitute thevarious layers such that the optical distance for the layers interposedbetween the metal electrode and the light-emitting layer satisfiesequation (5), but there is no limitation to this.

In particular, with a color conversion type color panel, the backlightis blue, and hence it is effective to use a metal having a largerreflection coefficient for blue than for red as the reflective metal;specifically, Zn, Mo or Cr may be used. Moreover, the transparentelectrically conductive film can be made blue by, for example, addingCuO, Co or Ti in an amount of not more than 1% to the oxide layerconstituting the transparent electrically conductive film.

In addition to the constitution shown in FIG. 1, an organic EL device ofthe invention may have the constitution shown in FIG. 2.

FIG. 2 is a drawing for explaining the structure in the case that thelower electrode of the organic EL device is made to be an anode; withthis constitution, a metal electrode 28, a transparent electricallyconductive film 27, which is an anode, a hole injection layer 23, a holetransport layer 22, a light-emitting layer 24, an electron transportlayer 25, an electron injection layer 26, and a transparent electrode21, which is a cathode, are formed in this order on a substrate 29.Here, regarding the constitution of the part comprising the electroninjection layer 26 and the transparent electrode 21 (the cathode), onecan envisage a structure in which the electron injection layer 26 isformed from an ultra-thin film of an alkali or alkaline earth metaloxide, fluoride, boride or chloride, an ultra-thin film of a metal suchas Al is deposited thereon, and an In₂O₃—ZnO oxide layer (IZO) isfurther provided thereon, or a structure in which a transparentelectrode 21 made of a transparent oxide such as IZO is directlydeposited on the electron injection layer 26.

In FIG. 2, the hollow arrows indicate light direction and reflection asin claim 1 and the thin, vertical, double-headed arrow indicates anoptical distance.

Note that in addition to organic EL devices having the layer structuresshown in FIGS. 1 and 2, the invention can also be applied to organic ELdevices having all other organic EL device constitutions proposedhitherto, for example a constitution in which a hole transport layer isnot provided.

EXAMPLE 1

FIG. 3 is a sectional view of a color conversion type color panelconstituted using an organic EL device of the invention. Cr (5 nm) andPt (100 nm) were deposited as a metal electrode 303 as reflective metalon a substrate 301 having TFTs 302 thereon, and then an In₂O₃—ZnO oxidelayer (IZO: refractive index 2.2) was further deposited as a transparentelectrically conductive film 304 acting as an anode on the metalelectrode 303. The metal electrode 303 used here as the reflective metalis not limited to being a laminate of Cr and Pt so long as it is anelectrically conductive metal or alloy having unevenness of not morethan 4 nm. Moreover, the formation of the IZO film was carried out bysputtering, but may instead be carried out using another film formationmethod such as electron beam vapor deposition or resistive heating vapordeposition.

A hole injection layer 305, a hole transport layer 306, and alight-emitting layer 307 were deposited in this order on the transparentelectrically conductive film 304 by resistive heating vapor deposition,and then an 8-hydroxyquinoline Al complex (Alq₃) was further formedthereon to a thickness of 20 nm as an electron transport layer 308.

A laminate 309 of an electron injection layer and an upper transparentelectrode was constituted by depositing LiF to a thickness of 0.5 nm asthe electron injection layer, then depositing 1 nm of Al and 220 nm ofIZO as the upper transparent electrode, and then finally depositing 300nm of SiON as a protective layer.

The optical distance for the organic EL device having this constitutionwas adjusted between the IZO transparent electrically conductive film304 that was the lower electrode (the anode), the hole injection layer305, the hole transport layer 306, and the Pt film in the metalelectrode 303. The wavelength of the light from the color conversiontype backlight was 470 nm, and the hole injection layer 305 wasdeposited to a thickness of 80 nm and the hole transport layer 306 to athickness of 20 nm; consequently, taking the refractive index of theorganic material to be 1.85, from the interference condition of equation(5) the IZO film thickness was made to be 183 nm. Furthermore, the IZOfilm that was the transparent electrically conductive film 304constituting the lower electrode was colored blue by adding 0.6% of CuOthereto.

A protective layer 316 was provided on the substrate 301 on which thedevice had been formed as described above, and this substrate and asubstrate 310 on which red, green and blue color-converting filters 311,312 and 313 had been formed in advance were put face to face, and thenwith a gap therebetween filled with a gel body 314, an outer peripheralportion of the device was sealed with an outer periphery sealant 315,thus completing the panel. Here, ‘color-converting filters’ are filtersprovided with fluorescent filters and/or color filters.

As a result of comparing the properties of the panel having theconstitution described in the present example with the properties of apanel having a conventional constitution, it was found that theefficiency of extracting out light could be improved from 2.0% to 3.0%,and hence the current passed at the same brightness could be reduced to⅔. Furthermore, a contrast ratio of 200:1 at 100 cd/m² under 1000 Lx wasobtained.

Moreover, upon carrying out similar comparative experiments withmonochrome and area color panels, similar results were obtained.

In FIG. 3, the hollow arrows indicate light direction.

EXAMPLE 2

The same comparison as in Example 1 was carried out but using In₂O₃—SnO₂(ITO) (refractive index 2.0) of film thickness 201 nm instead ofIn₂O₃—ZnO as the transparent electrically conductive film material; inthis case, similar effects to with Example 1 were again obtained. TheITO film can be deposited using a method such as sputtering, vapordeposition or CVD. Moreover, similar results were also obtained in thecase of using ZnO or SnO₂ as the transparent electrically conductivefilm material and adjusting the optical distance.

As described above, according to the invention, a transparentelectrically conductive film is provided on a surface of a metalelectrode of an organic EL device on the light-emitting layer side, andthe thickness of this transparent electrically conductive film isadjusted such that light reflected by the metal electrode is made toundergo interference and thus strengthen itself in the device, wherebythe external quantum efficiency can be improved with no accompanyingdeterioration in the brightness; furthermore, light of a specificwavelength is made to be absorbed by the metal electrode and thetransparent electrically conductive film, and hence the contrast isimproved; as a result, there can be provided an organic EL device and anorganic EL panel using the same, according to which the external quantumefficiency can be improved with no accompanying deterioration in thebrightness, and moreover the contrast can be improved.

1-10. (canceled)
 11. An organic electroluminescent device comprising an organic electroluminescent light-emitting part including an organic light-emitting layer, between a metal electrode and a transparent electrode, the organic electroluminescent device comprising: a transparent electrically conductive film on a surface of the metal electrode, on the organic electroluminescent light-emitting part side thereof; wherein a thickness of the transparent electrically conductive film is set so as to satisfy the following equation, where L is the optical distance from the organic light-emitting layer to the metal electrode, and λ is the wavelength of light emitted by the organic light-emitting layer: L=(2n+1)λ/4 (n=0, 1, 2, . . . ).
 12. The organic electroluminescent device according to claim 11, wherein a material of the transparent electrically conductive film is one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂.
 13. A monochrome panel or area color panel, including the organic electroluminescent device according to claim
 11. 14. An organic electroluminescent device comprising an organic electroluminescent light-emitting part including an organic light-emitting layer, between a metal electrode and a transparent electrode, the organic electroluminescent device comprising: a transparent electrically conductive film is provided on a surface of the metal electrode, on the organic electroluminescent light-emitting part side thereof; wherein light of wavelengths different than the wavelength of light emitted by the organic light-emitting layer is absorbed by at least one, or both, of the metal electrode and the transparent electrically conductive film, and only light of the wavelength emitted by the organic electroluminescent light-emitting layer is discharged from the transparent electrode.
 15. The organic electroluminescent device according to claim 14, wherein a material of the transparent electrically conductive film is one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂.
 16. A monochrome panel or area color panel, including the organic electroluminescent device according to claim
 14. 17. The organic electroluminescent device according to claim 14, wherein the organic electroluminescent light-emitting layer emits blue light, the metal electrode comprises Zn, Mo or Cr, or an alloy thereof, and the metal electrode absorbs blue light.
 18. A color conversion type color panel, comprising the organic electroluminescent device according to claim 17, a blue monochrome backlight, and color-converting filters, wherein light other than blue light is absorbed by the metal electrode, and only blue monochrome light from the backlight is reflected by the metal electrode.
 19. The organic electroluminescent device according to claim 14, wherein the transparent electrically conductive film has an impurity added thereto so as to be colored to a color the same as the color of the light emitted by the organic electroluminescent light-emitting layer.
 20. The organic electroluminescent device according to claim 19, wherein the organic electroluminescent light-emitting layer emits blue light, the transparent electrically conductive film is constituted from a material of one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂, containing an impurity of one of CuO, Co and Ti at a concentration of not more than 1%, and the transparent electrically conductive film absorbs blue light.
 21. A color conversion type color panel, comprising the organic electroluminescent device according to claim 20, a blue monochrome backlight, and color-converting filters, wherein light other than blue light is absorbed by the transparent electrically conductive film of the organic electroluminescent device, and only blue monochrome light from the backlight is reflected by the metal electrode.
 22. An organic electroluminescent device comprising an organic electroluminescent light-emitting part including an organic light-emitting layer, between a metal electrode and a transparent electrode, the organic electroluminescent device comprising: a transparent electrically conductive film on a surface of the metal electrode on the organic electroluminescent light-emitting part side; wherein a thickness of the transparent electrically conductive film is set so as to satisfy the following equation, where L is the optical distance from the organic light-emitting layer to the metal electrode, and λ is the wavelength of light emitted by the organic light-emitting layer: L=(2n+1)λ/4 (n=0, 1, 2, . . . ); and wherein light of wavelengths different than the wavelength of light emitted by the organic electroluminescent light-emitting layer is absorbed by the metal electrode and/or the transparent electrically conductive film, and only light of the wavelength emitted by the organic electroluminescent light-emitting layer is discharged from the transparent electrode.
 23. The organic electroluminescent device according to claim 22, wherein a material of the transparent electrically conductive film is one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂.
 24. A-monochrome panel or area color panel, including the organic electroluminescent device according to claim
 22. 25. The organic electroluminescent device according to claim 22, wherein the organic electroluminescent light-emitting layer emits blue light, the metal electrode comprises Zn, Mo or Cr, or an alloy thereof, and the metal electrode absorbs blue light.
 26. A color conversion type color panel, comprising the organic electroluminescent device according to claim 25, a blue monochrome backlight, and color-converting filters, wherein light other than blue light is absorbed by the metal electrode, and only blue monochrome light from the backlight is reflected by the metal electrode.
 27. The organic electroluminescent device according to claim 22, wherein the transparent electrically conductive film has an impurity added thereto so as to be colored to a color the same as the color of the light emitted by the organic electroluminescent light-emitting layer.
 28. The organic electroluminescent device according to claim 27, wherein the organic electroluminescent light-emitting layer emits blue light, the transparent electrically conductive film is constituted from a material of one of In₂O₃—ZnO, In₂O₃—SnO₂, ZnO and SnO₂, containing an impurity of one of CuO, Co and Ti at a concentration of not more than 1%, and the transparent electrically conductive film absorbs blue light.
 29. A color conversion type color panel, comprising the organic electroluminescent device according to claim 28, a blue monochrome backlight, and color-converting filters, wherein light other than blue light is absorbed by the transparent electrically conductive film of the organic electroluminescent device, and only blue monochrome light from the backlight is reflected by the metal electrode. 